This invention relates to a method of designing an LSI pattern formed on a specimen, such as a mask or a wafer, and more particularly to a pattern design method for making corrections in a design pattern with bent portions frequently used on an element region.
In recent years, semiconductor manufacturing technology has made a remarkable progress and LSIs whose minimum feature size is 0.35 .mu.m have been mass-produced. Such a miniaturization of LSIs has been realized as a result of a rapid progress in submicroscopic pattern forming techniques known as photolithography.
In photolithography, an exposure mask is first formed on the basis of a design pattern of an LSI. Then, with a projection exposure device, the pattern formed at the mask is projected onto the resist on a wafer, forming a resist pattern. Using the resist pattern as a mask, the underlying layer is etched, forming an LSI pattern on the wafer.
In the times when the pattern size was sufficiently larger than the limit resolution of an exposure device, the plane shape of an LSI pattern to be formed on a wafer was used as a design pattern. As patterns have been miniaturized, the pattern formed on a wafer has differed from the design pattern in a mask formed to the LSI design due to the influence of diffracted light at the exposure device. The difference between the patterns has begun to exhibit adverse effects obviously.
For example, there are many bent patterns in the gate patterns on an element region. When such bent patterns are transferred from the mask onto a wafer, the patterns are blurred due to optical proximity effects. As a result, the finished line widths at the positions corresponding to the bent portions are greater than the design dimensions. That is, the finished line widths at the position corresponding to the bent portions are larger than the desired dimensions. This causes variations in the operating speed of the elements, the threshold voltage, and the dynamic characteristics, including the source-drain current value. When a contact hole is next to bent patterns, the contact hole can come into contact with the gate, resulting in an insulation failure.
To eliminate such problems, a method has been proposed which adds very small auxiliary patterns to, for example, the corners of a design pattern to form a mask plan differing from a conventional design pattern and which forms a mask according to the mask design, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-242595 or Jpn. Pat. Appln. KOKOKU No. 62-7353.
Methods of this type, however, have the following problem: when very small auxiliary patterns are added to the corners of a bent pattern, the volume of data about the mask design becomes tremendous. The design data is converted into drawing data for exposure mask formation and is expressed as an aggregate of various figures. Therefore, when the volume of data has become tremendous as a result of an increase in the number of figures caused by the addition of auxiliary patterns, the computer takes a longer time to perform. processing, leading to an increase in the time required to draw a mask.
Furthermore, it is very difficult to control the size of an auxiliary pattern accurately. The reason is as follows. Because an electron beam or laser light used in mask drawing is a round beam, the corners of the auxiliary pattern are rounded. Therefore, when the dimensions of the auxiliary pattern are as small as about several tens of nanometers, the roundness of the corners has a great effect on the shape of the transferred pattern. As a result, the dimensions of the auxiliary pattern in the design data differ from the dimensions of the transferred auxiliary pattern. This makes it difficult to obtain the effect of correction as expected.
Moreover, in logic products, since the gate layers have various patterns ranging from isolated patterns to dense patterns, it is necessary to consider the effect the diffracted light from those patterns has on the bent patterns. It is very difficult to determine the size of an auxiliary pattern according to each pattern. The determination also needs a lot of labor.
As described above, in the prior art, when very small auxiliary patterns were added to the corners of a design pattern to form a bent pattern accurately, the volume of data about the mask design was increased. As a result, the computer needed a very long time to perform processing. In addition, it took a longer time to draw a mask. Furthermore, it was very difficult to control the size of an auxiliary pattern accurately.